1. Field of the Invention
The present invention relates to a lighting controller for a lighting device for a vehicle, and more particularly to a lighting controller for a lighting device for a vehicle constructed so as to control a semiconductor light source composed of a semiconductor light emitting element to be turned on.
2. Background Art
As the lighting device for a vehicle, a lighting device for a vehicle using a semiconductor light emitting element such as an LED (Light Emitting Diode) as a light source has been known. Mounted on such a lighting device for a vehicle is a lighting control circuit for controlling the LED to be turned on.
As the lighting control circuit, for instance, a lighting control circuit has been proposed in which the battery voltage of a vehicle is boosted and the boosted voltage is applied to LEDs to drive a light source having a plurality of LEDs connected in series (see Patent Document 1).
In such a lighting control circuit, a structure is employed in which a voltage not higher than the forward voltage (a voltage drop) of the LED is applied to the LED to supply a prescribed current to the LED. Thus, when a supply voltage is constant, a prescribed electric current can be always supplied to the LED.
However, during a transient time, for instance, at the time of starting by turning on a power switch, when the lighting control circuit performs a control for allowing a supply current to the LED to come close to a set value, if a control delay arises, the supply current to the LED exceeds the set value to overshoot so that an over-current may be supplied to the LED. Further, when a load suddenly changes, for instance, when a chattering phenomenon arises that, when a lead wire for connecting the lighting control circuit to the LED is disconnected from a contactor and then connected to the contactor again, because the load is open and accordingly a detected current is zero, the lighting control circuit carries out a control for increasing an output voltage as much as possible to maintain the detected current to the set value. When the output voltage of the lighting control circuit reaches a maximum value, if the LED as the load is connected to the lighting control circuit, the over-current may be possibly supplied to the LED. When the over-current is supplied to the LED, a bonding wire is disconnected or a chip is deteriorated due to a current concentration. Thus, the LED fails.
[Patent Document 1] JP-A-2004-51014.
To prevent the over-current from being supplied to the LED during the transient time, a method may be devised that a resistance element is inserted into a circuit for connecting the lighting control circuit to the LED to consume the current supplied during the transient time by the resistance element and prevent the over-current from being supplied to the LED. However, in this method, because the current is consumed by the resistance element even in a steady state, a power loss is increased.